Disallow su and sudo params in same play/task

```
---
- name: Test the ansible-playbook command's su support
  hosts: etcd
  su: yes
  sudo_user: testuser
  gather_facts: no

  tasks:
    - name: "Running /usr/bin/whoami"
      command: /usr/bin/whoami
```

```
14:19 $ ansible-playbook -i staging test-su-play.yml
sudo password:
ERROR: sudo params ("sudo", "sudo_user") and su params ("su", "su_user") cannot be used together
```